The present invention relates to plasma arc torches, and more particularly to an improved nozzle construction for initiating an arc in such torches utilizing a high voltage, high frequency signal applied to either an electrode or to the nozzle.
The starting of plasma torches has for a long time been a problem area in plasma torch development and the focus of much engineering attention.
There are currently three know methods to initiate a plasma arc discharge and start a plasma arc torch: 1) the high frequency discharge or its relative, the high voltage spark discharge, 2) contact starting, and 3) an exploding wire technique. In each method, an arc is drawn in an ionizable gas between a cathode (an electrode) and an anode (a nozzle or component of a nozzle).
The oldest, and most widely used method is the high frequency, high voltage spark discharge method. The high voltage, high frequency generates charge carriers which create an electrical current path in the gas in the gap between the cathode and the electrode to establish D.C. flow of current, a pilot arc discharge. It is common practice to attach the high frequency coil to the power supply line leading to the electrode or the nozzle, but on smaller systems having a lower power rating, e.g., those characterized by a DC amperage of 200 or less, the high frequency coil is usually coupled to the power line for the electrode.
An example of this high frequency, high voltage starting method is described in U.S. Pat. No. 3,641,308 to Couch, Jr., et al. A brief, high voltage pulse applied to the cathode initiates an arc across the gap to a nozzle which is connected through a switch and a resistor to ground. The workpiece is also grounded so that once the gas flow is initiated the arc will transfer from the nozzle to the workpiece. The switch is then opened so that the nozzle is electrically floating and the workpiece remains connected to ground. This general method of starting is also disclosed in U.S. Pat. Nos. 3,082,314 to Arata et al; 3,131,298 to Browning; 3,534,388 to Ito et al; 3,619,549 to Hogan et al; 3,787,247 to Couch, Jr.; 3,833,787 to Couch, Jr.; and 4,203,022 to Couch, Jr. et al.
In prior art plasma arc torches the electrode has traditionally had a generally cylindrical configuration, whether a cylindrical disk seated in a solid copper tube as described in the aforementioned U.S. Pat. Nos. 3,641,308, 4,203,022 or the electrode-nozzle arrangement shown and described in U.S. Pat. Nos. 4,421,970; 4,791,268; and 4,861,962. In these prior art arrangements, the lower end of the electrode adjacent the nozzle typically has a cylindrical configuration. The immediately opposite nozzle surface typically mirrors the outer configuration of the electrode, or is smooth, conical and downwardly converging. In both instances the nozzle includes a central exit port where the plasma arc exits the torch and attaches to the workpiece. It is significant that while the transition between the interior nozzle surface and the exit port may be a sharp corner, and while this corner may be closely spaced from the electrode, it is not located in a region where it is closer to the electrode than immediately adjacent portions of the nozzle.
There are two principal problems with the prior art designs. First, when the arc is initiated it is sometimes difficult to induce the breakdown of the gas to start the plasma arc discharge. To reliably reach the breakdown potential to initiate the arc, the voltage level applied to the electrode is increased. This, however, accentuates the electromagnetic noise interference problems associated with all high-frequency, high-voltage starting arrangements.
Another quite significant problem is the lack of reliability as to where on the electrode and nozzle the arc will initiate. This lack of control over the location of the arc causes wear problems. For example, if the high frequency spark begins high on the side of the electrode (away from the exit port), by the time it travels to the lower portion of the electrode, typically containing an electron emitting element, the DC current and voltage levels can be ramped up to a level such that double arcing occurs. As is well known in the trade, double arcing will quickly destroy torch components such as the nozzle and the electrode.
In the prior art it is known to machine mark the electrode to facilitate breakdown of the gas at the mark on start up. However, to the best of applicant's knowledge, there has been no nozzle structure specifically designed to reduce the breakdown potential (and the elapsed time required to achieve breakdown) and to control the location of where the pilot arc is initiated by applying a high frequency, high voltage to either the nozzle or the electrode. This is particularly true where the high voltage, high frequency coil is attached to the power line leading to the nozzle, not the electrode. In this situation with prior art constructions, variations in the external configuration of the electrode would have little or no effect on the location of the arc.
It is therefore a principal object of the present invention to provide a nozzle construction or a plasma arc torch which reliably initiates an arc within a small, well-defined annular region of the electrode and the nozzle.
Another principal object is to reduce electrode wear as compared to comparable prior art nozzles in comparable torches operated under the same conditions.
Yet another principal object of this invention is to reduce the breakdown potential required to initiate an arc discharge in a given plasma arc torch.
A further object in the present invention is to provide a nozzle construction which reduces electromagnetic interference during the high frequency, high voltage start up with other electrical and electronic components in the operating area.
A still further of the present invention is to provide a nozzle construction with the foregoing advantages which is simple in construction, has a comparatively low cost of manufacture, and can be used as a replacement part for conventional nozzles of existing plasma arc torches.